Developing highly active and durable electrocatalysts for oxygen evolution reaction (OER) are required for efficient hydrogen production by polymer electrolyte membrane water electrolysis (PEMWE). While RuO₂ shows the highest OER activity among pure transition metal oxides catalysts, the low durability hinders its practical application as OER catalyst for PEMWE. Recent studies demonstrated that RuO₂ mixed with other metal oxides, such as IrO₂ and SnO₂ etc. showed higher activity and durability than pure RuO₂ (1-3). When the RuO₂ deposited on the other metal oxides, interfacial resistance and lattice strain which derives from the lattice mismatch should influence on the OER properties. In this study, to clarify the influence of interface between different rutile-type oxides, we fabricated RuO₂/MO₂/TiO₂(110) (M = Ir or Sn) single crystal model catalyst and evaluated the oxygen evolution activity and durability.

Experimental

Nb-doped TiO₂(110) (Nb: 1 wt%) single crystal substrate was etched in 20 % HF solution for 10 min and annealed at 1000 ℃ in air. After the sample transfer to the APD chamber, 2 nm-thick Ir or Sn was deposited onto the substrate at room temperature under 0.5 Pa-O₂ partial pressure and annealed at 350 ℃ for 30 min under the same O₂ pressure. After that, 4 nm-thick Ru was deposited on the MO₂/TiO₂(110) and was annealed under the same condition of the MO₂ layer. Hereafter, the sample was denoted as RuO₂/MO₂/TiO₂.

Oxygen evolution activity was measured by linear sweep voltammetry (LSV) at a scan rate of 1 mV/s in N₂-purged 0.1 M HClO₄ at room temperature. The electrochemical impedance spectroscopy was conducted at 1.5 V vs. reversible hydrogen electrode (RHE) to measure the resistance of solution (R_s), electrodes (R_e), and charge transfer for OER (R_ct). The LSV curves were corrected using the measured R_s. Chronopotentiometry was conducted at 0.5 mA/cm² for 120 min to evaluate the durability. In-plane XRD analysis was performed for estimating lattice strain in the surface RuO₂ layer.

Results and Discussions

XRD analysis showed that both of the MO₂ interlayer and RuO₂ catalyst layer grow with (110) orientation on the TiO₂(110) substrate. Figure 1(a) shows initial LSV curves for OER. The onset potential of RuO₂/SnO₂/TiO₂ samples clearly shifted to lower potential side relative to the RuO₂/IrO₂/TiO₂ and RuO₂/TiO₂ samples. The overpotential at 0.1 mA cm^-2 of the RuO₂/SnO₂/TiO₂ (b) was ca. 25 mV lower than the other two samples. The chronopotentiometric curves (c) showed the overpotential of RuO₂/TiO₂ increased by 250 mV after 120 min. On the other hand, change in the overpotential of the RuO₂/SnO₂/TiO₂ was only 25 mV, indicating the high durability.

Cole-cole plots at 1.5 V (d) revealed that both of the R_e and R_ct were significantly reduced owing to SnO₂ interlayer, while the IrO₂ layer slightly increased the R_e and decreased the R_ct. In-plane XRD results showed the compressive lattice strain was induced in [001] direction in RuO₂ layer on TiO₂ substrate and decreased by insertion of IrO₂ and SnO₂ layer. However, the lattice spacing in [-110] direction was almost the same for all the samples. Figure 1 (e) shows the relation between R_ct and the lattice strain in [001] direction. The R_ct tended to decrease with decreasing the compressive strains, suggesting the OER activity of RuO₂(110) surface is related to the anisotropic strains in [001] direction. The results demonstrated that the activity and durability of RuO₂/TiO₂ catalysts can be improved by insertion of SnO₂ layer at the catalyst/substrate interface.

Acknowledgments

This study was partly supported by JSPS KAKENHI Grant Number 21H01661 and Toyota Mobility Foundation Hydrogen Initiative.

References

1. R. V. Genova-Koleva, F. Alcaide, G. Álvarez, P. L. Cabot, H.-J. Grande, M. V. Martínez-Huerta and O. Miguel, Journal of Energy Chemistry, 34, 227 (2019).
2. J. Li, X. Qin, G. Chen and G. Chen, Journal of the Taiwan Institute of Chemical Engineers, 125, 186 (2021).
3. R. Samanta, P. Panda, R. Mishra and S. Barman, Energy & Fuels, 36, 1015 (2022).